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582 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 34, NO. 2, JANUARY 15, 2016 Dense Space-Division Multiplexed Transmission Systems Using Multi-Core and Multi-Mode Fiber Takayuki Mizuno, Member, IEEE, Hidehiko
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582 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 34, NO. 2, JANUARY 15, 2016 Dense Space-Division Multiplexed Transmission Systems Using Multi-Core and Multi-Mode Fiber Takayuki Mizuno, Member, IEEE, Hidehiko Takara, Member, IEEE, Akihide Sano, Member, IEEE, and Yutaka Miyamoto, Member, IEEE (Invited Paper) Abstract In this paper, we describe recent progress in spacedivision multiplexed (SDM) transmission, and our proposal and demonstration of dense space-division multiplexing (DSDM), which offers the possibility of ultra-high capacity SDM transmission systems with high spatial density and spatial channel count of over 30 per fiber. We introduce the SDM transmission matrix, which cross indexes the various types of multi-core multi-mode transmissions according to the type of light propagation in optical fibers and how the spatial channels are handled in the network. For each category in the matrix, we present the latest advances in transmission studies, and evaluate their transmission performance by spectral and spatial efficiencies. We also expound on technologies for multi-core and/or multi-mode transmission including optical fiber, signal processing, spatial multi/demultiplexer, and amplifier, which will play key roles in configuring DSDM transmission systems, and review the first DSDM transmission experiment over a 12 core 3 mode fiber. Index Terms Digital signal processing (DSP), optical communication systems, optical fibers, optical fiber communication, space division multiplexing (SDM), spectral efficiency, wavelength division multiplexing (WDM). I. INTRODUCTION WITH the rapid increase in Internet traffic, demand for much higher capacity will increase in optical communication networks to accommodate future high definition videos and new data communication services. Fig. 1 shows the growth in transmission capacity per optical fiber as mentioned in research studies as well as commercial optical communication systems [1], [2]. Until the 1980s, time division multiplexing was studied actively. It uses an electric multiplexing technique. Gigabit/s class transmission capacity was realized by modulating optical signals at high speeds. Then, in the mid-1990s, wavelength division multiplexing (WDM) in combination with optical amplification techniques was employed for capacity expansion by multiplexing optical signals with different wavelengths in the C and L bands. Advances in WDM technology allowed the multiplexing of a large number of wavelengths of Manuscript received June 23, 2015; revised August 12, 2015 and August 31, 2015; accepted September 2, Date of publication September 27, 2015; date of current version February 5, This work used a part of results from the research commissioned by the National Institute of Information and Communications Technology of Japan. The authors are with the NTT Network Innovation Laboratories, NTT Corporation, Yokosuka , Japan ( lab.ntt.co.jp). Color versions of one or more of the figures in this paper are available online at Digital Object Identifier /JLT Fig. 1. Transmission capacity per optical fiber in research and commercial systems. over 30. With 30 wavelength channels and 100-GHz spacing, the bandwidth is 3 THz, which is in agreement with the amplification range of an Erbium-doped fiber amplifier (EDFA) in the C-band. Such system was denoted as dense wavelength division multiplexing (DWDM), and terabit/s class capacity was realized with DWDM technology. In the late 2000s, digital coherent technology, which once was studied extensively before the emergence of EDFAs, regained its appeal with the emergence of large-scale integrated circuits and digital signal processing (DSP) technology. Recent digital coherent studies greatly improve the spectral efficiency by using multi-level modulation formats and high-performance compensation in optical fiber transmission lines. As a result, the transmission capacity per fiber has reached 100 Tb/s in research, and 10 Tb/s in commercial systems. The above three major technologies have led to increases in optical fiber transmission capacity by a factor of more than times over the past three decades. As the capacity is growing at an annual rate of 1.4 times, and is anticipated to grow at an even faster rate, research and development continues to target larger capacity. However, around the year 2020, transmission capacity will supposedly reach the theoretical limit over a single-mode fiber (SMF) of around 100 Tb/s [3], [4]. This is due to the nonlinear effect and the limit of power transmissible through a single-mode core [5]. To overcome these limits, the additional use of the spatial dimension [1] has attracted a lot of research interest in recent years. Various space division multiplexed (SDM) transmission IEEE. Translations and content mining are permitted for academic research only. Personal use is also permitted, but republication/redistribution requires IEEE permission. See standards/publications/rights/index.html for more information. MIZUNO et al.: DENSE SPACE-DIVISION MULTIPLEXED TRANSMISSION SYSTEMS USING MULTI-CORE 583 transmission schemes reported, and Section III evaluates the efficiencies of using spectrum and space resources by each category in the matrix. Section IV presents the latest technologies for multi-core and/or multi-mode transmission including optical fiber, multiple-input and multiple-output (MIMO) signal processing, spatial multi/demultiplexer (MUX/DEMUX), and briefly elaborates on topics for future studies. Finally, Section V reviews the world s first demonstration of DSDM transmission, and Section VI summarizes the main contents and concludes the paper. Fig. 2. Spectral efficiency versus spatial multiplicity of SDM-WDM transmission experiments (N: Number of cores, M: Number of modes). schemes have been explored and have demonstrated their potential by setting milestones in transmission capacity [6], [7] and capacity distance product [8], [9] per fiber. Fig. 2 shows the relationship between spectral efficiency and spatial multiplicity of recent SDM-WDM transmission experiments. The spatial multiplicity is the total number of spatial channels multiplexed in cores or modes of a fiber, and excludes polarizations. The tilted dotted line represents the aggregate spectral efficiency, which is the product of the spatial multiplicity (horizontal axis) and spectral efficiency (vertical axis). The spatial multiplicity in early transmission experiments started from M =3in multimode transmission [10] and N =7in multi-core transmission [11], [12], where M is the number of modes, and N is the number of cores. Soon, the multiplicity was increased to the current maximum of N =19[13], yet spatial multiplicity values of over twenty remained unexplored. In order to increase scalability, we need to raise the multiplicity to the region we refer to as dense SDM (DSDM) region [14] with a spatial multiplicity of over 30. Further development of SDM technology was required to make a step forward and realize DSDM with higher spatial density and spatial multiplicity. We have developed fundamental technologies for both multi-core and multi-mode transmission, and have verified, for the first time, that it is possible to further increase the spatial multiplicity of a fiber into the DSDM region, opening the first step toward DSDM transmission systems [14]. DSDM, our proposal and demonstration, has been addressed by several research groups, and followup DSDM experiments on multi-core few-mode fiber (MC-FMF) were conducted a year later [15], [16]. This paper examines the latest SDM transmission experiments, and describes DSDM with a spatial multiplicity of 30 and higher, which we proposed and demonstrated for the first time utilizing both multi-core and multi-mode in transmission [14]. Similar to DWDM with dense wavelength spacing and high count of over several tens of wavelength channels [17], [18], we have shown that DSDM with high spatial density and large spatial multiplicity is effective in offering expanded scalability with the use of the spatial dimension. The paper is organized as follows. Section II introduces the SDM transmission matrix to describe the various types of SDM II. SDM TRANSMISSION MATRIX SDM transmission generally focuses on the optical fiber used as the transmission medium; either it is few-mode fiber (FMF) or multi-core fiber (MCF). From the viewpoint of transmission, signal processing and optical network application, there are diverse transmission types in SDM. We introduce the SDM transmission matrix to summarize the various SDM transmission schemes [19] in Fig. 3. The entries identify the type of spatial channel transmission. The columns divide the state of light propagation along the fiber, namely, (I) single-mode in uncoupled single-mode core, (II) super-mode in coupled single-mode cores, or (III) multi-mode in multimode/few-mode cores. The rows distinguish the transmission fiber having (A) multiple spatial channel groups or (B) single spatial channel group. Transmissions in category A are the parallel form of category B, and contain multiple spatial channel groups. While the term spatial superchannel describes data streams of subchannels, which consist of separate modes/cores occupying the same wavelength, spatial channel groups describes the smallest unit of cores/modes that can be transmitted together in SDM networks. For example, in the case of category IIB and IIIB transmission, spatial channels in coupled cores or a multi-mode core are coupled, and signal processing is required after long-haul transmission to separate the spatial channels. We regard the channels as belonging to the same spatial channel group. Spatially multiplexed tributary signals in the same spatial channel groups are routed together, and are received and processed at the same destination in a network. At the receiver of each destination, MIMO signal processing will be used to separate the coupled channels from the spatial channel group. The spatial channel groups can be transmitted, added, and dropped individually in the optical domain in network nodes. We briefly summarize the latest transmission schemes for each category in the matrix. IA. Uncoupled Multi-Core Transmission: This category, often regarded as multi-core transmission, has been studied actively from an early stage of SDM research. Various MCFs have been developed for this class of transmission, including 7, 12, and 19 core fibers. Initial transmission experiments using seven-core multiplicity reached the capacity equivalent to the SMF limit of around 100 Tb/s [11], [12]. Next, record transmission experiments were reported. 305 Tb/s capacity per fiber was achieved, which is approximately triple the SMF limit, by using 19-core multiplicity [13]. Further, an ultra-high capacity of more than 584 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 34, NO. 2, JANUARY 15, 2016 Fig. 3. SDM transmission matrix to organize various SDM transmission schemes. IA: multi-core, IIA: groups of coupled-core, IIB: coupled-core, IIIA: multi-core multi-mode, and IIIB multi-mode transmission. Typical fiber cross-sectional diagram is included for each category. 1 Pb/s was achieved for the first time over a 52-km lowcrosstalk one-ring structured 12-core fiber with polarizationdivision multiplexed (PDM) 32-quadrature amplitude modulation (32 QAM) and an aggregate spectral efficiency of 91.4 b/s/hz [6]. The highest capacity distance product of 1 Eb/s km was achieved with 7 core [8] and 12 core [9] MCFs at transmission distances of 7326 and 1500 km, respectively. These transmission records were made possible in conjunction with the development of low crosstalk MCFs whose characteristics of each core are equivalent to those of an SMF, low-loss and low crosstalk fan-in/fan-out (FI/FO) devices, and multi-core amplifiers. Overviews of these technologies will be provided in Section III. IIB. Coupled-Core Transmission: This category also uses an MCF as the transmission line but the signals in different cores are designed to couple with each other and form super-modes. The cores of the MCF can be arranged at a smaller spacing than that in category IA because their coupling is desired. However, as for multi-mode transmission, MIMO signal processing is required to uncouple the signals at the receiver. Three coupled-core transmission over 4200 km [20] and six coupled-core transmission over 305 km [21] have been reported. IIA. Groups of Coupled-Core Transmission: This category is the parallel form of category IIB, where the transmission fiber contains groups of coupled-cores. The cores in the same spatial channel group will couple with each other and form supermodes, but the cores of different spatial channel groups are isolated. So far, an experiment on three groups of three coupledcores over 715 km has been reported [22]. Spatial multiplicity can be further increased by adding more groups of coupled-cores in the fiber s cross-sectional area. IIIB. Multi-Mode Transmission: Transmission research in this category has also been popular yielding the multiplexing of three or six spatial modes in a FMF [23] [25], or mode group division multiplexing in a conventional graded-index (GI) multimode fiber (MMF) [26]. FMFs and various types of mode MUX/DEMUX are now commercially available. Mode division multiplexing of a few modes over a conventional GI-MMF [27] and photonic bandgap fiber [28] have also been demonstrated. Modal impairments like differential mode delay (DMD) and mode dependent loss (MDL) greatly impact transmission performance. Using a GI type refractive index profile and a fiber management technique that combines multiple FMFs with positive and negative DMD values are common techniques for suppressing the maximum absolute DMD. The maximum transmission capacity is currently 57.6 Tb/s with three mode transmission over a total of 119 km FMF [23]. The longest distance for the three-mode transmission of WDM PDM quadrature phase shift keying (QPSK) signals is 1000 km [25]. The maximum six mode transmission distance is currently 177 km with WDM PDM-16 QAM signals [24]. Most recently, transmission of 15 mode PDM QPSK modulated signals with b/s/hz aggregate spectral efficiency over a 22.8 km MMF has been presented [29]. IIIA. Multi-Core Multi-Mode Transmission: This category is the parallel form of category IIIB. The spatial multiplicity scales with the product of the number of cores and modes and so provides efficient scalability. However, transmission in this category was difficult until present because problems in both category IA and in IIIB transmissions had to be solved simultaneously. We have already developed the technologies required for the simultaneous use of multi-core and multi-mode, and successfully demonstrated the first transmission in this category [14]. Utilizing the multiplier effect of the multiple modes in multiple cores, we expanded the spatial multiplicity to 36 (12 cores 3 modes), and transmitted 20 DWDM 36 DSDM signals modulated at PDM-32QAM over 40.4 km. The aggregate spectral efficiency was b/s/hz and potential capacity is around triple that of the 1 Pb/s achieved in [6] and [7]. Additional studies in this category were reported from several organizations. They are 7 core 3 mode transmission over 1 km [30], 36 core 3 mode 5.5 km fiber [15], and 19 core 6 mode MIZUNO et al.: DENSE SPACE-DIVISION MULTIPLEXED TRANSMISSION SYSTEMS USING MULTI-CORE 585 Fig. 4. Spatial and spectral efficiencies of SDM-WDM transmission experiments from various categories of the SDM transmission matrix. transmission over 9.8 km [16]. However, the transmission distances were all limited to short reach. We have recently demonstrated the long distance DSDM transmission of PDM-QPSK signals over 527 km [31], which is more than ten times that of our original DSDM transmission [14]. The above are fundamental transmission schemes for SDM fiber, and there may be combinations of several categories such as mixtures of single-mode and multi-mode in a transmission fiber, or multi-mode coupled-core transmission. From the optical fiber perspective, transmission in categories IA, IIA, and IIB use MCF, category IIIB uses FMF/MMF and category IIIA uses MC-FMF. From the optical components viewpoint, category IA transmission can use well-established conventional single-mode equipment, amplifiers, and devices. However, category II and III transmission require additional devices to handle the multi-modes, and the optimum number of modes or super-modes for transmission is under investigation. In terms of signal processing, transmission in category II and III requires MIMO signal processing, in particular, for long haul and high capacity transmission, and reducing DSP complexity is essential. Transmission studies are underway, and the potential for each category remains to be exploited. III. SPATIAL AND SPECTRAL EFFICIENCIES The important metrics for evaluating SDM transmission performance are the efficiencies of use in terms of space and spectrum. The SDM fibers used in each transmission scheme in the previous section have different cladding diameters. Therefore, to make a useful efficiency comparison considering the space needed for spatial channel transmission in a fiber, we introduce the parameter of spatial efficiency, η spatial, defined as the spatial multiplicity divided by fiber cross-sectional area: Spatial multiplicity η spatial = Cross sectional area. (1) The above spatial multiplicity includes the total number of spatial modes and cores used for transmission, but excludes polarization. Fig. 4 shows spectral efficiency η spectral per core/mode versus spatial efficiency η spatial of the fibers used in some recent SDM-WDM studies. As a reference, the vertical dotted line shows the spatial efficiency (η spatial =81.5(1/mm 2 ))ofthe conventional SMF. The plot visualizes the current performance of each category of the SDM transmission schemes in terms of efficiencies. The transmission capacity per fiber is given as: Capacity = Net data rate n WL Spatial multiplicity = (η spectral Bandwidth) (η spatial Fiber area) (2) where n WL is the number of WDM channels, and bandwidth is the product of n WL and frequency spacing. The bandwidth used for transmission is mainly determined by the characteristics of the light source and amplification technique being used, and the fiber cross sectional area is determined by the fiber design and fabrication techniques. Thus, to realize ultra-high capacity, increasing η spectral and η spatial are essential by advancing DWDM and DSDM transmission technologies, respectively. The uncoupled multi-core transmission, category IA, has higher η spatial than conventional SMF, and the value increases with the number of cores N. However, as we increase N, intercore crosstalk will increase, and it becomes more difficult to transmit high order modulation signals over long distances, resulting in a lower η spectral. If we are to realize DSDM- WDM transmission over a single-mode MCF transmission media, higher scaling technology is required that can increase both η spatial and η spectral. Coupled MCF transmission, category IIB, has higher η spatial with fewer cores than category IA because of the smaller core pitch. However, its η spectral is lower than that of uncoupled multi-core transmission at present, and extensive research is needed to improve η spectral. Multi-mode transmission, category IIIB, can increase η spatial in proportion to the number of modes by spatial mode-multiplexing within the 125 μmdiameter cladding. Its parallel form, the multi-core multi-mode transmission in category IIIA, provides both the highest η spatial and η spectral. This result suggests that it is possible to transmit data spatially efficiently at higher capacities via the multi-core multi-mode approach. 586 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 34, NO. 2, JANUARY 15, 2016 Fig. 5. Example configuration of a DSDM transmission system. The system includes
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